2 potential problems.
1. thickness - you know the length and width, but how about height? there are a lot of cloths out that claim to be microfiber but they seem like a very towel. If it is too thick, it may not be able to close, or even worse, damage the hinge over time. If it is strong enough to have a tag sewn onto it and sustain machine washing "time after time," it may be on the thicker side.
2. serves NO purpose - so if you put a barrier between the GLASS (not the screen itself) and keyboard, the oils will remain on the keys. by the auction's own admission, grease and oil are removed by adding a little bit of water. What makes that different from any other cloth or even a paper towel?
If you leave the oils on the keyboard long enough, they might become permanent. (i say "might" because i haven't seen keyboard wear like that since the Pismo/Lombard keyboards and it could be from oil or just plain wear and tear)
In essence, you're not correcting or even avoiding the problem, you're just enabling it. You're just putting the newspapers over the vomit/spilt milk (a la "Big Daddy"). The keyboard cover will prevent the oils from getting on the keys in the first place.